Implantable medical devices (IMDs) include devices designed to be implanted into a patient. Some examples of these devices include cardiac function management (CFM) devices such as implantable pacemakers, implantable cardioverter defibrillators (ICDs), cardiac resynchronization devices, and devices that include a combination of such capabilities. The devices can be used to treat patients using electrical or other therapy or to aid a physician or caregiver in patient diagnosis through internal monitoring of a patient's condition. The devices may include one or more electrodes in communication with one or more sense amplifiers to monitor electrical heart activity within a patient, and often include one or more sensors to monitor one or more other internal patient parameters. Other examples of implantable medical devices include implantable diagnostic devices, implantable drug delivery systems, or implantable devices with neural stimulation capability.
Additionally, some IMDs detect events by monitoring electrical heart activity signals. In CFM devices, these events can include heart chamber expansions or contractions. By monitoring cardiac signals indicative of expansions or contractions, IMDs can detect abnormally slow heart rate, or bradycardia. The IMDs can detect abnormally rapid heart rate, or tachyarrhythmia. Ventricular tachyarrhythmia can be terminated with high energy shock therapy using an ICD. However, cardioversion/defibrillation therapy can cause patient discomfort and consumes a relatively large amount of battery power which may lead to a shortened useful device lifetime. Ventricular tachyarrhythmia can be treated with anti-tachycardia pacing (ATP). ATP uses lower energy than cardioversion/defibrillation therapy and may painlessly terminate an episode of ventricular tachyarrhythmia. Therefore it is sometimes desirable to treat ventricular arrhythmias with ATP rather than a shock.